The invention relates to an electric lamp comprising:
a lamp vessel sealed in a vacuum-tight manner and consisting of glass having an SiO.sub.2 content of at least 95% by weight, PA1 an electric element arranged inside the lamp vessel, PA1 current supply conductors extending through the wall of the lamp vessel to the electric element, PA1 at least one current supply conductor made of molybdenum with a continuous coating of glass having an SiO.sub.2 content of at least 95% by weight, which coating forms with the current supply conductor a glass/metal interface and is fused to the lamp vessel, while PA1 the surface of the coating encloses with the coated surface of the current supply conductor at the points at which they meet an angle .alpha..
Such a lamp is known from GB 602,215 (1948.5.21).
In substantially all types of electric lamps comprising a lamp vessel of glass having an SiO.sub.2 content of at least 95% by weight, the current supply conductors are passed in a vacuum-tight manner through the wall of the lamp vessel in that the current supply conductors comprise a foil-shaped part of molybdenum which is embedded in a pinched seal of the lamp vessel. In this construction the foil-shaped part, which is only from approximately 15 to approximately 100 .mu.m thick and has etched knife-edge rims, must be connected to a conductor extending into the interior of the lamp vessel and to a conductor extending from the pinched seal to the exterior, for which purpose welding connections must be made. The ohmic resistance of the foil-shaped part leads not only to electric losses, but also to a detrimental heat generation in the pinched seal. The current supply conductor, moreover, is a slack assembly, which can be manipulated only with difficulty during the manufacture of the lamp and which makes it difficult to position accurately in the lamp vessel that part which is to be located within said lamp vessel. The accuracy of positioning could be improved if the current supply conductor with a foil-shaped part could also within the lamp vessel be held and continuously positioned during the manufacture of a first pinched seal of the lamp vessel. A rigid current supply conductor would then have to be used for the manufacture of a second seal. Another disadvantage of lamps having a pinched seal is that the seal is destroyed at a comparatively low gas pressure of about 80 bar. In spite of these disadvantages, pinched seals are generally used in commercially available lamps. Short-arc discharge lamps are exceptions in this respect.
In short-arc discharge lamps, a construction is used in which a tungsten current supply conductor is sealed into glass having a comparatively high coefficient of expansion, which is connected via glasses having coefficients of expansion stepwise decreasing to the glass of the lamp vessel, which has a very low coefficient of expansion. This so-called "graded seal" obtained with the use of so-called "transition glasses" is expensive and can only be realized manually in most cases. Moreover, the construction occupies a large amount of space.
The construction having a foil-shaped part is used because glasses having an SiO.sub.2 content of at least 95% by weight, such as, for example, quartz glass and "Vycor" glass, i.e. a glass containing 96% by weight of SiO.sub.2, have a linear coefficient of expansion which is considerably smaller (in the range from about 4.times.10.sup.-7 K.sup.-1 to about 12.times.10.sup.-7 K.sup.-1) than that of molybdenum (about 55.times.10.sup.-7 K.sup.-1). This great difference in coefficient of expansion and the great difference between the softening temperatures of the glasses and the operating temperature of the lamps on the one hand and room temperature on the other hand result in that molybdenum cannot be included in a vacuum-tight manner in these glasses without special steps being taken. Thanks to the foil shape, a vacuum-tight seal can be obtained with molybdenum owing to the ductility of this material in spite of the large difference in thermal expansion.
For several decades attempts have been made to devise special measures by which molybdenum current supply conductors in the form of wire or tube could be sealed into glasses, such as quartz glass. The result of these efforts is that commercially available lamps in such glasses still have either a pinched seal with an embedded metal foil or a graded seal with a tungsten current supply conductor.
The construction according to the aforementioned GB 602 215 is not used either. According to this Patent Specification, a molybdenum conductor is heated at its outer surface in an inert or reducing atmosphere by passage of an electric current or by a heat source inside the conductor, if the latter is hollow, after which it is provided with a quartz glass coating. It has been found difficult to realize the described construction in a reproducible manner. The reproducibility is found to be connected to the degree to which a coating, for example of quartz glass, can be obtained on the current supply conductors in a reproducible manner, which coating adheres to these conductors and encloses an angle .alpha. of max. 90.degree. in order to prevent fractures of the coating.
U.S. Pat. No. 4,086,075 discloses a method of providing a vitreous coating on metal wires. The method consists in that a metal wire together with a glass tube tightly fitting around it is heated in a high-frequency field in a protective gas, such as nitrogen. The high-frequency field may be produced by a coil connected to a current source. A non-shortcircuited coil is present in the high-frequency field, which coil is heated, as is the metal wire, by the high-frequency field. They both heat the glass tube to its melting point. The coated wire is free from oxides and impurities have not been able to accumulate between the wire and the coating. The adhesion of a vitreous coating to a tungsten conductor apparently requires that the coating is provided on a tungsten conductor which is free from adsorbed gases, oxides and other impurities at its surface.